1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, to a backlight assembly having fluorescent lamps and light emitting diodes as a light source and a liquid crystal display device including the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite alignment as a result of their long, thin shapes and are arranged to have initial pre-tilt angles. The alignment direction can be controlled by applying an electric field. Specifically, variations in an applied electric field influence the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image that has a desired brightness can be produced.
In general, a liquid crystal display (LCD) device includes a liquid crystal panel and a backlight assembly. The liquid crystal panel includes two substrates, which are spaced apart and face each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode, and the electrodes of each substrate also face each other. The backlight assembly is disposed at a rear side of the liquid crystal panel and provides the liquid crystal panel with a light. A voltage is applied to each electrode, and an electric field is induced between the electrodes. An arrangement of the liquid crystal molecules is changed by varying the intensity of the electric field.
Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
The backlight assembly may be classified into a side-type and a direct-type depending on a position of a light source. Generally, the backlight assembly includes a lamp as the light source. The lamp may be a cold cathode fluorescent lamp (CCFL) or an exterior electrode fluorescent lamp (EEFL). The side-type backlight assembly includes one lamp (or two lamps) at a side of the liquid crystal panel. The side-type backlight assembly further includes a light guide plate to provide light from the lamp to the liquid crystal panel by refracting the light. The direct-type backlight assembly includes a plurality of lamps directly under the liquid crystal panel to provide light to the liquid crystal panel.
Since the direct-type backlight assembly has relatively high brightness, the direct-type backlight assembly is widely used for large televisions or monitors.
FIG. 1 is a perspective view illustrating a liquid crystal display (LCD) device including a direct-type backlight assembly according to the related art. The LCD device includes a liquid crystal panel 10, a backlight assembly 20, and several combining elements.
In FIG. 1, the backlight assembly 20 is disposed at a rear side of the liquid crystal panel 10, and a main support 40, which is a rectangle frame, edges the backlight assembly 20 and the liquid crystal display panel 10. The main support 40 is combined with a bottom cover 50 thereunder to maintain a shape and to minimize a loss of light. To unite the above elements, a top cover 60 is connected to the main support 40 and the bottom cover 50.
Driving integrated circuits are connected to at least one side of the liquid crystal panel 10 through flexible printed circuit boards 12, and the flexible printed circuit boards 12 are bent toward a side of the main support 40 or a rear side of the bottom cover 50.
The backlight assembly 20 includes a reflecting sheet 22, a plurality of lamps 24, a couple of side supports 26, and optical sheets 28. The reflecting sheet 22 covers an inner surface of the bottom cover 50, and the plurality of lamps 24 are disposed on the reflecting sheet 22 side by side. The plurality of lamps 24 may be cold cathode fluorescent lamps. The side supports 26 are set on the bottom cover 50 across both ends of the plurality of lamps 24 to thereby fix the plurality of lamps 24. The optical sheets 28 are disposed over the plurality of lamps 24.
Accordingly, light, which is directly emitted from the plurality of lamps 24 or reflected on the reflecting sheet 22 after emitted from the plurality of lamps 24, is properly changed while passing through the optical sheets 28 and is provided to the liquid crystal panel 10.
Recently, light emitting diodes (LEDs) have been used as the light source. A backlight assembly including the LEDs may be referred to as an LED backlight assembly.
FIG. 2 is a perspective view illustrating an LCD device including an LED backlight assembly according to the related art. In the LCD device of FIG. 2, the same parts as the LCD device of FIG. 1 have the same references as the LCD device of FIG. 1, and explanation for the same parts will be omitted.
In the LED backlight assembly 30, a plurality of LEDs 33 are used as the light source. The plurality of LEDs 33 are set tip on each of a plurality of metal core printed circuit boards (MCPCBs) 32 in a line, wherein the plurality of MCPCBs 32 have a stripe shape and are arranged on the inner surface of the bottom cover 50. The plurality of LEDs 33 include one of red, green and blue LEDs, and on each MCPCB 32, the red, green and blue LEDs are sequentially arranged, whereby red, green and blue lights are mixed to produce a white light.
A reflecting sheet 34 covers the plurality of MCPCBs 32. The reflecting sheet 34 has a plurality of through-holes 35, which correspond to the plurality of LEDs 33, respectively, and thus the plurality of LEDs 33 protrude through the plurality of through-holes 35 over the reflecting sheet 34, respectively. Optical sheets 38 are disposed over the plurality of LEDs 33, and a transparent window 36 is disposed between the plurality of LEDs 33 and the optical sheets 38. The transparent window 36 includes reflecting dots 37 corresponding to the plurality of LEDs 33, respectively. If there is no transparent window 36 including the reflecting dots 37, dots are shown on images due to strong light directly emitted from the plurality of LEDs 33. The reflecting dots 37 weaken the light directly emitted from the plurality of LEDs 33. Accordingly, light from the plurality of LEDs 33 is provided to the liquid crystal panel 10 after the light is distributed by the reflecting dots 37 and then is properly changed while passing through the optical sheets 38.
In practice, the above-mentioned backlight assemblies have problems. More particularly, while the direct-type backlight assembly including the fluorescent lamps does not require additional elements such as the transparent window or the reflecting dots, the direct-type backlight assembly including the fluorescent lamps is disadvantageous in achieving the high brightness and uniformity as compared with the LED backlight assembly. Furthermore, the direct-type backlight assembly including the fluorescent lamps needs inverter circuits for driving respective fluorescent lamps to thereby cause an increase of manufacturing costs. On the other hand, the LED backlight assembly has relatively high and uniform brightness and good reproducibility in comparison with the direct-type backlight assembly including the fluorescent lamps, and the inverter circuits are not required. However, since the LED backlight assembly has dot light sources, the reflecting dots and the transparent window are needed to distribute light from the dot light sources. This causes an increase in a thickness of the LED backlight assembly.
Trials and researches for a hybrid-type backlight assembly, which includes the fluorescent lamps and the light emitting diodes, have been proposed, and there exists problems due to characteristics and different structures of each light source.